This invention relates to a fuel supply control method for internal combustion engines, and more particularly to a method of this kind in which cutting-off of the fuel supply to the engine is effected at appropriate times so as to avoid deterioration of the driveability of the engine, burning of an exhaust gas-purifying catalyst provided in the engine, etc. which would otherwise be caused by cutting-off of the fuel supply to the engine.
In internal combustion engines in general, the supply of fuel to the engine is cut off (hereinafter abbreviated as "fuel cut") while the engine is decelerating in a predetermined operating condition, so as to improve the fuel consumption and emission characteristics of the engine as well as burning of a catalyst provided in the exhaust system of the engine for purifying the exhaust gases.
However, if the fuel cut is frequently effected, there can occur fluctuations in the driving torque of the engine, impeding smooth operation of the engine and giving an unpleasant feeling of shock to the driver or the passenger in the vehicle.
In an attempt to avoid such disadvantages and obtain smooth operation of the engine at decelerating fuel cut, it has been proposed by Japanese Provisional Patent Publication (Kokai) No. 56-50232 to delay effecting the fuel cut by a predetermined period of time after fulfillment of a fuel cut condition, and it has also been proposed by Japanese Provisional Patent Publication (Kokai) No. 54-45423 to delay effecting the fuel cut while the engine is operating in a predetermined speed region.
However, according to the former proposal, the fuel cut is delayed by the same predetermined period of time irrespective of the rotational speed of the engine. As a result, if the delaying period of time is set at a large value in order to avoid an unpleasant feeling of shock at fuel cut at a low rotational speed of the engine, a number of times of fuel injections take place before the lapse of the delaying period of time at a high rotational speed of the engine, which results in supply of an excessive amount of fuel to the engine, causing burning of the catalyst due to reaction with an excessive amount of unburned fuel in the exhaust gases. On the other hand, according to the latter proposal, when the engine is operating in a high speed region, fuel cut is effected immediately upon fulfillment of a predetermined fuel cut condition without delay. This reduces the possibility of burning of the catalyst. However, from the view point of improving the driveability through mitigation of a shock at fuel cut in a high engine speed region, it is not desirable to effect the fuel cut immediately upon fulfillment of the fuel cut condition without delay when the engine is operating in the high engine speed region. Furthermore, protection of the catalyst from burning will be possible by setting the delaying period of time within a range of values at which there is little possibility of burning of the catalyst at high rotational speeds of the engine.